In a high-speed (e.g., 100 GbpS) optical transmission system, a digital coherent optical transceiver that has an optical waveguide substrate configured such that various optical elements are provided on a semiconductor substrate is used (see, for example, Japanese Laid-open Patent Publication No. 2014-157181). It is known that an optical waveguide provided on an optical waveguide substrate is doped with erbium (Er) and the erbium-doped optical waveguide is used as an amplifier (see, for example, Japanese Laid-open Patent Publication No. 2006-74016 and Japanese Laid-open Patent Publication No. 2003-283012). An optical waveguide substrate configured such that various optical elements are provided on a semiconductor substrate is also called a planar waveguide. The planar waveguide is a substrate on which optical elements such as a polarization-beam splitter (PBS) and an optical 90-degree hybrid circuit are provided. Use of a planar waveguide configured such that a plurality of elements can be provided on a single substrate allows a reduction in size of a digital coherent optical transceiver including an optical receiver.
A digital coherent optical transceiver can be further reduced in size and power consumption by using light output from a light source as transmission light and using, as local light, part of the light split by using a beam splitter. Use of part of the transmission light as local light keeps light output power of the transmission light from decreasing, and therefore the transmission light is amplified by an amplifier such as an erbium-doped fiber amplifier (EDFA) after being modulated by an optical modulator. The EDFA includes an erbium-doped optical fiber having a length of several meters and having a core doped with an erbium ion (Er+3). The EDFA is an amplifier that amplifies light in a 1.55 μm band by exciting an erbium ion (Er+3) by using excitation light. The transmission light output from the light source is modulated by the optical modulator and is then multiplexed with the excitation light and amplified by the EDFA. In this way, desired light amplification characteristics are achieved.
However, an erbium-doped optical fiber included in an EDFA typically has a length of several meters. It is therefore not easy to reduce a size of a digital coherent optical transceiver that amplifies transmission light by using an EDFA. In view of such circumstances, it is desirable that an optical module has a small size and has good light amplification characteristics.